Nowadays, thin film transistors have been widely used as pixel switches of a liquid crystal display. Polycrystalline silicon thin film transistors with higher carrier mobility may have a higher potential to monolithically integrate peripheral drive circuits on a display panel. As a key process for a method for fabricating polycrystalline silicon thin film transistors, a process of forming a polycrystalline film is very important. Due to a requirement of a low temperature process, a common method for forming a polycrystalline film comprises steps of: depositing an amorphous silicon film on a host substrate, and then transforming amorphous silicon into polycrystalline silicon by means of an appropriate crystallization process. One important crystallization process is laser recrystallization, which makes a part of the amorphous silicon with a certain thickness melted instantaneously by short-time laser irradiation and then solidified rapidly so as to achieve the purpose of recrystallization. However, disadvantages of the laser recrystallization lie in the fact that, because a nucleation process of a silicon film in a molten state is random during a solidification process, it is hard to control a size of a grain, and it is also hard to precisely control an energy density of a laser because a process window is very narrow.